1. Field of the Invention
The present invention relates to a flexible printed circuit board for an electronic, or electrical device such as a mobile phone or a lap top computer, for transmitting and receiving an electric signal between a main body and a second body such as a display which is pivotable to the main body.
2. Description of the Related Art
A flexible printed circuit board is a micro circuit printed on a flexible plastic material sheet, and is developed in response to the recent trend for small-sized, light-weight, and complicated electronic or electrical apparatuses. The flexible printed circuit board can be distributed by high density and three dimensional wire, is highly durable against repeated bending, has no wire distribution error, and can be easily assembled. Therefore, the flexible printed circuit board is widely used for various areas such as a camera, a computer, a peripheral device, a cellular phone, video and audio devices, a camcorder, a printer, a DVD device, a TFT LCD, satellite equipment, military equipment and medical equipment.
FIG. 1 is a view of an example of a cellular phone to which a general flexible printed circuit board is applied. As shown, in the cellular phone 1, a main body 2 and a folder 3 are foldably jointed by a hinge 4. A flexible printed circuit board 10 connects a main board 5 housed in the main body 2 and a sub board 6 housed in the folder 3 for operating an LCD to transmit and receive an electric signal.
Referring to FIG. 2, the flexible printed circuit board 10 comprises a first end part 11 connected with the main board 5 and a second end part 12 connected with the sub-board 6, and at least one horizontal part 13 exists in the hinge 4 between the first end part 11 and the second end part 12. The first end part 11 and the second end part 12 are approximately perpendicular with the horizontal part 13.
The folder 3 can be pivotably closed and opened to the cellular phone main body 2 by the flexible printed circuit board 10 such that an electric signal can be efficiently received and transmitted between the main board 5 and the sub board 6.
When the folder is pivotably closed and opened to the cellular phone main body 2 as described above, repeated bending and twisting simultaneously occurs in the flexible printed circuit board 10. In specific, the bending occurs at the perpendicular part of the flexible printed circuit board 10, and the twisting occurs at the horizontal part 13. Generally, the flexible printed circuit board 10 is robust to the bending but susceptible to the twisting.
When the twisting occurs at the horizontal part 13 as shown in FIG. 2, a stress is generated responsive to the tensile deformation as the edge is extended longer than the actual length. The tensile stress causes fatigue failure such that a minute circuit of the flexible printed circuit board 10 is disconnected and damaged.
To prevent the disconnection and damage of the minute circuit of the flexible printed circuit board 10 by the tensile stress, conventionally, the horizontal part 13 of the flexible printed circuit board 10 was extended longer than required in view of the twisting such that the tensile deformation may be minimized. If this method is used, however, the elongated horizontal part 13 of the flexible printed circuit board 10 sags such that the horizontal part 13 is frictionized with the inside of the hinge 4 and abraded.